The Fischer-Tropsch process provides for the processing of synthesis gas, which comprises hydrogen and carbon monoxide, to yield high molecular weight hydrocarbons. Steam reforming is one process that provides for the processing of desulfurized hydrocarbon feedstocks, such as natural gas, lower molecular weight hydrocarbons, or naphtha, to yield synthesis gas that may be used as a feed to a Fischer-Tropsch synthesis process.
In the steam reforming process, the desulfurized hydrocarbon feedstock is mixed with steam and passed over a suitable catalyst, e.g., a catalyst comprising nickel on an alumina support, at an elevated temperature and pressure to yield a synthesis gas. The synthesis gas comprises hydrogen and carbon monoxide at concentrations providing for particular hydrogen-to-carbon monoxide molar ratios (H2/CO ratio).
In the Fischer-Tropsch process, the synthesis gas is fed into a reactor where it is converted over a suitable catalyst, e.g., cobalt or iron, preferably, cobalt, supported on alumina, silica, or titania, at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to or more than 200 carbon atoms. The product distribution of the Fischer-Tropsch synthesis is influenced by the H2/CO ratio of the synthesis gas feed due to typical selectivity characteristics of the Fischer-Tropsch catalysts. Reactions with lower H2/CO ratios are more selective for yielding C5+ hydrocarbon molecules, and reactions with higher H2/CO ratios are less selective for yielding C5+ hydrocarbon molecules.
In a process that integrates a reforming step that yields a synthesis gas with a Fischer-Tropsch synthesis step that reacts the synthesis gas to yield high molecular weight hydrocarbons, it can be desirable to control the H2/CO ratio of the synthesis gas feed to the Fischer-Tropsch synthesis step. By controlling the H2/CO ratio of Fischer-Tropsch feed, the Fischer-Tropsch product properties can, to a certain extent, be controlled to provide a desired Fischer-Tropsch product mix.
Thus, it is desirable to have a process that includes a unit for reforming hydrocarbon feeds and a means for controlling the H2/CO ratio of the synthesis gas feed to a Fischer-Tropsch unit.